Although receptive field sizes account for some of the differences in visual sensitivity across the retina, the sensitivity at a given retinal location can also vary. The human eye can process information over an enormous range of luminance (about twelve (12) log units). The visual system changes its sensitivity to light; a process called adaptation, so that the eye can detect the faintest signal on a dark night and yet not be overloaded by the high brightness of a summer beach scene. Adaptation involves four major processes:
1. Changes in Pupil Size. The iris constricts and dilates in response to increased and decreased levels of retinal illumination. Iris constriction has a shorter latency and is faster (about 0.3 s) than dilation (about 1.5 s). There are wide variations in pupil sizes among individuals and for a particular individual at different times. Thus, for a given luminous stimulus, some uncertainty is associated with an individual's pupil size unless it is measured. In general, however, the range in pupil diameter for young people may be considered to be from two (2) mm for high levels to eight (8) mm for low levels of retinal illumination. This change in pupil size in response to retinal illumination can only account for a 1.2 log unit change in sensitivity to light. Older people tend to have smaller pupils under comparable conditions.
2. Neural Adaptation. Neural adaptation is a fast (less than one (1 s) second) change in sensitivity produced by synaptic interactions in the visual system. Neural processes account for virtually all the transitory changes in sensitivity of the eye where cone photopigment bleaching has not yet taken place (discussed below)—in other words, at luminance values commonly encountered in electrically lighted environments, below about 600 cd/m2. Because neural adaptation is so fast and is operative at moderate light levels, the sensitivity of the visual system is typically well adjusted to the interior scene. Only under special circumstances in interiors, such as glancing out a window or directly at a bright light source before looking back at a task, will the capabilities of rapid neural adaptation be exceeded. Under these conditions, and in situations associated with exteriors, neural adaptation will not be completely able to handle the changes in luminance necessary for efficient visual function.
3. Photochemical Adaptation. The retinal receptors (rods and cones) contain pigments which, upon absorbing light energy, change composition and release ions which provide, after processing, an electrical signal to the brain. As previously stated, there are believed to be four photopigments in the human eye, one in the rods and one each in the three cone types. When light is absorbed, the pigment breaks down into an unstable aldehyde of vitamin A and a protein (opsin) and gives off energy that generates signals that are relayed to the brain and interpreted as light. In the dark, the pigment is regenerated and is again available to receive light. The sensitivity of the eye to light is largely a function of the percentage of unbleached pigment. Under conditions of steady brightness, the concentration of photopigment is in equilibrium; when the brightness is changed, pigment is either bleached or regenerated to reestablish equilibrium. Because the time required to accomplish the photochemical reactions is finite, changes in the sensitivity lag behind the stimulus changes. The cone system adapts much more rapidly than does the rod system; even after exposure to high levels of brightness, the cones will regain nearly complete sensitivity in ten (10 min) minutes—twelve (12 min) minutes, while the rods will require sixty (60 min) minutes (or longer) to fully dark-adapt.
4. Transient Adaptation. Transient adaptation is a phenomenon associated with reduced visibility after viewing a higher or lower luminance than that of the task. If recovery from transient adaptation is fast (less than one (1 s) second), neural processes are causing the change. If recovery is slow (longer than one (1 s) second), some changes in the photopigments have taken place. Transient adaptation is usually insignificant in interiors, but can be a problem in brightly lighted exteriors where photopigment bleaching has taken place. The reduced visibility after entering a dark movie theater from the outside on a sunny day is an illustration of this latter effect.
Scotopic and photopic vision is well known. As light levels decrease, the human eye responds more to bluer light and less to yellow/red light. With age, the eye also loses transmission of blue light and therefore benefits from more blue-light energy. The intent of this scotopic phosphor blend is to address both of these conditions with a phosphor that enhances human vision.
Accordingly, there is a need for a scotopic after-glow lamp (fluorescent lamp with a non-uniform phosphor blend of scotopic enhanced phosphors and after-glow phosphors) as part of an emergency lighting system. A combination of phosphor blends has been developed to be used in the scotopic after-glow lamp. The present invention is critical to an emergency lighting system to include the use of light bulbs made with a scotopically rich primary phosphor coat combined with a strontium aluminate after-glow base phosphor. The eye's ability to respond to the after-glow available light could be critical to a person's ability to react in an emergency situation. The primary scotopic phosphor blend of the present invention prepares the eye to respond and adapt quickly to the after-glow light if the lamp power is turned off and/or even if the lamp breaks.